When I want to change my Youtube profile picture, it says it's linked to my Google+ account and wants me to change it there.
But it's really not linked, not in the way you'd assume.
You have two choices if you want transparent background:
1) Have transparency on Google+, then on Youtube you will get an ugly gray picture background.
2) Have a white background on Youtube that matches the website background color, then you won't have transparency on Google+.
Thinking about it, while this sucks and is the usual corporate insanity, it's easy for me to make a choice.
And that's why I now have white background on Google+ (and thus also on my blog).
This is how corporate business creates progress. By removing stuff that works and adding stuff that doesn't work.